1. Field of the Invention
This invention relates to coatings for polymeric substrates, and more particular to adherent, environmentally stable, optically clear, abrasion resistant, glass coatings for polymeric substrates.
2. Description of the Prior Art
Plastic ophthalmic lenses have become increasingly popular recently due to their excellent optical properties, ease of fabrication, impact resistance, light weight, etc. One serious disadvantage, nevertheless, to plastic ophthalmic lenses has been their susceptibility to abrasion, particularly compared to traditional glass lenses.
Much research has been devoted to providing coatings for plastic lenses to improve their abrasion resistance. An example of one type of coating intended to solve this problem is given in U.S. Pat. No. 3,637,416, to Misch et al. The coatings described in the Misch et al. patent are applied by coating a plastic lens with a bonding or coupling film comprising an organic silicon compound and also coating it with a silica or silica gel.
Another chemical coating designed to improve the abrasion resistance of plastic lenses is given in Krekeler, U.S. Pat. No. 3,713,880. The Krekeler coatings are formed by coating lenses with a solution comprising a mixture of an alkyl silicate, a trifunctional or bifunctional organosilane and an antistatic additive in an organic solvent, followed by solvent evaporation.
A still further chemical-type coating for plastic lenses is described in Crandon et al., U.S. Pat. No. 3,700,487. The Crandon et al. coatings consist of lightly cross-linked polyvinyl alcohols coated onto polycarbonate lens substrates. It is stated that these polyvinyl alcohol coatings provide antifogging characteristics and are also abrasion resistant.
To date, however, none of the chemical coatings has proven totally successful as evidenced by their lack of commercial acceptance. This is probably because of the stringent optical and mechanical property requirements of such coatings.
A very desirable material for coating plastic lenses to increase their abrasion resistance is, of course, glass. The difficulty in forming successful glass coatings on polymeric substrates becomes readily apparent, however, when one considers the many differences between the substrate and coating. For example, there are chemical differences which tend to cause poor adhesion inasmuch as the substrate is an organic polymeric material and the coating is an inorganic, amorphous material. Also, there are significant physical differences, such as the thickness of the glass coating compared to that of the substrate, and even more importantly, the coefficients of thermal expansion of these different materials.
One attempt to provide glass coatings on polymeric substrates is described in U.S. Pat. No. 3,713,869, issued to Geffcken et al. The technique described in the Geffcken et al. patent involves the formation of a delicate intermediate layer formed by producing a glow discharge in a low-molecular organic vapor such as silicic-acid-methyl.
Another attempt at forming glass coatings on plastic substrates is described in Dietzel et al., U.S. Pat. No. 3,522,080. This patent relates to a process of depositing at least 50 successive layers of SiO.sub.x onto a substrate under high vacuum conditions and in the presence of oxygen.
Still another attempt to vacuum deposit glass coatings onto polymer substrates is disclosed in U.S. Pat. No. 3,645,779 to Kienel. This patent teaches the advantages of using as a starting material boron oxide-silicon oxide glass containing less than 5 percent by weight of sodium oxide.
Despite the extensive amount of prior research devoted to providing suitable abrasion resistant chemical coatings, and the more recent efforts to provide glass coatings, there is still a great need for a simple and direct technique for applying thin, adherent, abrasion resistant, environmentally stable, glass coatings to polymeric substrates, particularly plastic lenses.